1. Field of Invention
The present invention relates generally to a method and apparatus for smoothing surfaces having fine irregularities, of the order of a few microns and up to about 200 microns in size. In particular, the method and apparatus of the invention is useful in planarizing or polishing the surfaces of semiconductor wafers, precision optical lenses and the like using an ion beam in the presence of a radio frequency generated plasma.
2. Description of The Related Art
With advances in technology, there is increasingly a demand, in certain technologies, for precision planarization or polishing of surfaces for a variety of purposes. For example, highly polished precision surfaces are needed in optics and in the fabrication of semiconductors, using silicon wafers. Indeed, as explained below, requirements in the two technical fields are frequently interrelated, in that optical instruments are used in the manufacture of semiconductors.
The fabrication of very large scale integrated circuits on silicon wafers is a process requiring extreme precision because of the very fine details of the circuits. Indeed, the width of circuit lines are continually decreasing as the technology advances, and is now in the range of 0.18-100 microns. Since the circuits are produced using photolithographic techniques, extremely accurately ground lenses are required to provide fine detail. As a necessary consequence of providing such precise focusing, the lenses lack depth of field, i.e., an image is accurately produced only at a specific distance from the lens, and any deviation in this distance produces an increasingly unfocused or fuzzy image. Therefore, the surface onto which the image is projected must be as perfectly planar as possible to eliminate out-of-focus image fuzziness. A failure to maintain planarity exacerbates the problem as additional layers are formed on the nonplanar surface resulting in an increased proportion of defective semiconductors that must be rejected.
At present, the standard technique for restoring surface planarity, after certain processes that lay down component layers on the wafer, is chemical mechanical planarization (CMP). During this process, the surface of the wafer is polished with a polishing pad, and in certain instances with a chemical slurry, to remove excess material and also to replanarize the surface. When a slurry is used, it contains an abrasive, such as silica or alumina, and chemical additives that are designed to selectively react with and soften the composition of those components that must be planarized or removed from the wafer surface. Chemical slurries are not always used, and certain planarization and polishing steps may be carried out using a "fixed abrasive" in which an abrasive substance or composition is fixed in a polishing pad or other substrate that is used to polish the wafer surface.